Replicate snapshots from one Cinder backend to another

(smcginnis): Marking obsolete as this has been sitting out there for a long time. If this is still needed, please submit a new bp.

Define a volume driver method to optimize replication of a snapshot between two volume
backends controlled by the volume driver.

Why replicate snapshots rather than volumes?

* Snapshots are already quiescent. There is no extra work required to ship a consistent set of bytes
   to the destination. In fact, many volume drivers will implement volume replication by first creating
   a snapshot of the volume.
* Snapshots can be organized in a sequence, and the content of a new snapshot can be
   transferred more efficiently when expressed as a delta from a shared older snapshot.
* When replicating volumes it is frequently desirable to replicate at least the most recent N
   snapshots at the same time.
* Migrating a volume can be done more efficiently by first migrating snapshots, especially
  when the backends support incremental snapshots, while keeping the source volume alive.
  You simply iterate through shorter-duration incremental snapshots before finally quiescing
   the source volume.

Why snapshots rather than backups?

* Snapshots will be in a format chosen by the backend vendor, and hence optimized for that
   environment. Backups are formatted to be stored as a single blob in an object service that
   understands nothing about their structure or meaning.

Default algorithms will be used when replicating snapshots across different volume-drivers,
or when a volume driver feels that there is no optimization for it to perform.

snapshot_replicate (snapshot,source_server,destination_server[,force_full_copy])

   snapshot: the snapshot to be replicated.
   source_Server: the volume server where the snapshot currently is found.
   destination_server: the volume server where the snapshot is to be created.
   force_full_copy: optional parameter that suppresses use of vendor-specific
      options which would optimize the replication based upon prior replication
      of earier snapshots for the same object.

As with the other replicate methods, this method may yield an external-activity.